pre-metric systems were perfectly sensible in the time they were made for, it's when we started standardizing things and building precise machinery that they stopped making sense.
In the past there just wasn't as much of a need for it, you don't need standardized units and measurements to simply make something that functions, and when sharing measurements with others you can either get by with a vague "about 3 foot 4 inches" or you can literally give them the measurement on a physical object.
And when working like that it's actually nice to have these kinds of units, an adult thumb is roughly the same no matter who and most people have at least one of them on hand.
just
halfa'sounds like completely normal british english, like cuppa